This invention relates to digital loop carrier transmission systems.
In a typical digital loop carrier transmission system, such as Subscriber Loop Carrier (SLC.RTM.) systems, digital transmission takes place between a local digital switch and a remote terminal. The customer lines, or loops, couple customer premises equipment, such as standard telephones, to the remote terminal through channel units located at the remote terminal.
Due to variations in the lengths of the loops, it has been recognized that it is often necessary to add a certain amount of loss to the signals between the remote terminal and the customer equipment in both the off-hook and on-hook states in order to protect against too loud a signal for the shorter loops. In the case of the on-hook state, the standard approach has been to add a constant amount of loss to all customers coupled to the remote terminal. This has been effective in the past since there was a minimal amount of reflection back to the switch. While such an approach has worked for most systems, problems have arisen, for example, in systems employing caller identification routines. In such systems, frequency shift key (FSK) signals are delivered to the customer equipment in the on-hook state. Because of the high impedance of the customer equipment, all or most of the incoming signal is reflected back toward the switch. Due to the poor trans-hybrid loss of the central office terminal (COT) channel unit and/or significant reflective coefficient of the switch, the reflected signal is, in turn, reflected back to the remote terminal where it can interfere with the next incoming bit and produce an error. Adding a constant on-hook loss to all loops does not alleviate this condition since the combination of channel unit loss and cable loss could exceed the maximum loss allowable (13 dB) for some loops, and a low on-hook loss would not ensure caller ID reliability.